First and most obvious is the total destruction of the fertility of Middle and Lower Egypt, the conversion of that part of the valley into a desert, and the extinction of its imperfect civilization, if not the absolute extirpation of its inhabitants. This is the calamity threatened by the Abyssinian princes and the ferocious Portuguese warrior, and feared by the Sultans of Egypt. Beyond these immediate and palpable consequences neither party then looked; but a far wider geographical area, and far more extensive and various human interests, would be affected by the measure. The spread of the Nile during the annual inundation covers, for many weeks, several thousand square miles with water, and at other seasons of the year pervades the same and even a larger area with moisture by infiltration. The abstraction of so large an evaporating surface from the southern shores of the Mediterranean could not but produce important effects on many meteorological phenomena, and the humidity, the temperature, the electrical condition and the atmospheric currents of North-eastern Africa might be modified to a degree that would sensibly affect the climate of Europe.
The Mediterranean, deprived of the contributions of the Nile, would require a larger supply, and of course a stronger current, of water from the Atlantic through the Straits of Gibraltar; the proportion of salt it contains would be increased, and the animal life of at least its southern borders would be consequently modified; the current which winds along its southern, eastern, and north-eastern shores would be diminished in force and volume, if not destroyed altogether, and its basin and its harbors would be shoaled by no new deposits from the highlands of inner Africa.
In the much smaller Red Sea, more immediately perceptible, if not greater, effects, would be produced. The deposits of slime would reduce its depth, and perhaps, in the course of ages, divide it into an inland and an open sea, the former of which, receiving no supply from rivers, would, as in the case of the northern part of the Gulf of California, soon be dried up by evaporation, and its whole area added to the Africo-Arabian desert; the waters of the latter would be more or less freshened, and their immensely rich marine fauna and flora changed in character and proportion, and, near the mouth of the river, perhaps even destroyed altogether; its navigable channels would be altered in position and often quite obstructed; the flow of its tides would be modified by the new geographical conditions; the sediment of the river would form new coast-lines and lowlands, which would be covered with vegetation, and probably thereby produce sensible climatic changes.
Diversion of the Rhine.
The interference of physical improvements with vested rights and ancient arrangements, is a more formidable obstacle in old countries than in new, to enterprises involving anything approaching to a geographical revolution. Hence such projects meet with stronger opposition in Europe than in America, and the number of probable changes in the face of nature in the former continent is proportionally less. I have noticed some important hydraulic improvements as already executed or in progress in Europe, and I may refer to some others as contemplated or suggested. One of these is the diversion of the Rhine from its present channel below Ragatz, by a cut through the narrow ridge near Sargans, and the consequent turning of its current into the Lake of Wallenstadt. This would be an extremely easy undertaking, for the ridge is but twenty feet above the level of the Rhine, and hardly two hundred yards wide. There is no present adequate motive for this diversion, but it is easy to suppose that it may become advisable within no long period. The navigation of the Lake of Constance is rapidly increasing in importance, and the shoaling of the eastern end of that lake by the deposits of the Rhine may require a remedy which can be found by no other so ready means as the discharge of that river into the Lake of Wallenstadt. The navigation of this latter lake is not important, nor is it ever likely to become so, because the rocky and precipitous character of its shores renders their cultivation impossible. It is of great depth, and its basin is capacious enough to receive and retain all the sediment which the Rhine would carry into it for thousands of years. [Footnote: Many geographers suppose that the dividing ridge between the Lake of Wallenstadt and the bed of the Rhine at Sargans is a fluviatile deposit, which has closed a channel through which the Rhine anciently discharged a part or the whole of its waters into the lake. In the flood of 1868, the water of the Rhine rose to the level of the railway station at Sargans, and for some days there was fear of the giving way of the barrier and the diversion of the current of the river into the lake.]
Improvements in North American Hydrography.
We are not yet well enough acquainted with the geography of Central Africa, or of the interior of South America, to conjecture what hydrographical revolutions might there be wrought; but from the fact that many important rivers in both continents drain extensive table-lands, of moderate elevation and inclination, there is reason to suppose that important changes in the course of those rivers might be accomplished. Our knowledge of the drainage of North America is much more complete, and it is certain that there are numerous points within our territory where the courses of great rivers, or the discharge of considerable lakes, might be completely diverted, or at least partially directed into different channels.
The surface of Lake Erie is 565 feet above that of the Hudson at Albany, and it is so near the level of the great plain lying east of it, that it was found practicable to supply the western section of the canal, which unites it with the Hudson, with water from the lake, or rather from the Niagara which flows out of it. The greatest depth of water yet sounded in Lake Erie is but two hundred and seventy feet, the mean depth one hundred and twenty. Open canals parallel with the Niagara, or directly towards the Genesee, might be executed upon a scale which would exercise an important influence on the drainage of the lake, if there were any adequate motive for such an undertaking. Still easier would it be to enlarge the outlet for the waters of Lake Superior at the Saut St. Mary—where the river which drains the lake descends twenty-two feet in a single mile—and thus to produce incalculable effects, both upon that lake and upon the great chain of inland waters which communicate with it.
The summit level between the surface of Lake Michigan at its mean height and that of the River Des Plaines, a tributary of the Illinois, at a point some ten miles west of Chicago, is but ten and a half feet above the lake. The lake once discharged a part or the whole of its waters into the valley of the Des Plaines. A slight upheaval, at an unknown period, elevated the bed of the Des Plaines, and the prairie between it and the lake, to their present level, and the outflow of the lake was turned into a new direction. The bed of the Des Plaines is higher than the surface of the lake, and in recent times the Des Plaines, when at flood, has sent more or less of its waters across the ridge into the bed of the South Branch of Chicago River, and so into Lake Michigan.
A navigable channel has now been cut, admitting a constant flow of water from the lake, by the valley of the Des Plaines, into the Illinois. The mean discharge by this channel does not much exceed 23,000 cubic feet per minute, but it would be quite practicable to enlarge its cross-section indefinitely, and the flow through it might be so regulated as to keep the Illinois and the Mississippi at flood at all seasons of the year. The increase in the volume of these rivers would augment their velocity and their transporting power, and, consequently, the erosion of their banks and the deposit of slime in the Gulf of Mexico, while the opening of a communication between the lake and the affluents of the Mississippi, unobstructed except by locks, and the introduction of a large body of colder water into the latter, would very probably produce a considerable effect on the animal life that peoples them. The diversion of water from the common basin of the great lakes through a new channel, in a direction opposite to their present discharge, would not be absolutely without influence on the St. Lawrence, though probably this effect might be too small to be readily perceptible. [Footnote: From Reports of the Canal Commissioners of the State of Illinois, and especially from a very interesting private letter from William Gooding, Esq., an eminent engineer, which I regret I have not space to print in full, I learn that the length of the present canal, from the lake to the River Illinois, is 101 miles, with a total descent of a trifle more than 145 feet, and that it is proposed to enlarge this channel to the width of one hundred and sixty feet, with a minimum depth of seven, and to create a slack-water navigation in the Illinois by the construction of five dams, one of which is already completed. The descent for the outlet of the canal at La Salle on the Illinois to the Mississippi is twenty-eight feet, the distance being 230 miles. The canal thus enlarged would cost about $16,000,000, and it would establish a navigation for vessels of 1,200 to 1,500 tons burden between Lake Michigan and the Mississippi, and consequently, by means of the great lakes and the Welland canal, between the St. Lawrence and the Gulf of Mexico.]